1. Field of the Invention
The present invention relates to an optical modulator used in an optical communication system, an optical exchange apparatus, an optical information processing apparatus, or an optical recording apparatus, and a method of modulating light using the same.
2. Related Background Art
A conventional high-speed optical modulator utilizing a semiconductor element which can be driven with a low drive voltage and can be easily integrated with other optical electronic elements such as a semiconductor laser is known.
For example, an absorption type optical modulator utilizing an absorption end shift of a semiconductor (e.g., a semiconductor layer of a bulk or quantum well structure) upon application of an electric field is proposed in Applied Physics Letters, Vol. 47, pp. 1148-1150 (1985). A directional coupler type optical modulator utilizing a change in refractive index upon application of an electric field is proposed in Report OQE 86-39, the Institute of Electronic and Communication Engineers of Japan, (1986) is also proposed.
The former optical modulator comprises a semiconductor waveguide having a p-i-n structure. Upon application of an electric field, an absorption end is shifted by the Franz-Keldysh effect or QCSE (Quantum Containment Stark Effect), as shown in FIG. 3, to change the absorption index, and a transmittance of light having a given wavelength can be controlled In the optical modulator of this type, a wavelength used must be set closer to the absorption end so as to increase an extinction ratio. For this reason, a transmittance in a transmission state is decreased, and hence an insertion loss is increased. Light components having all wavelengths cannot always be modulated depending on the wavelength of the absorption end. That is, a wavelength to be modulated, i.e., the wavelength used, is limited to a predetermined range, resulting in inconvenience.
FIG. 1A is a schematic plan view showing an arrangement of a first conventional directional coupler type optical modulator, and FIG. 1B is a sectional view of the optical modulator along the line A--A' in FIG. 1A. This optical modulator comprises ridges 100, aluminum electrodes 101, waveguides 102, aluminum electrodes 103, an n.sup.- -GaAs layer (optical waveguide layer) 104, an n.sup.+ -GaAs layer 105, an aluminum electrode 106, and a GaAlAs layer 107.
In the above optical modulator, the electrodes are formed in coupling regions of the two waveguides, and an electric field is applied to these electrodes to cause a change in refractive index, thereby shifting a light wave between the waveguides.
FIG. 2A is a schematic plan view showing an arrangement of a second conventional directional coupler type optical modulator, and FIG. 2B is a sectional view of the optical modulator along the line A--A' in FIG. 2A. This optical modulator comprises optical waveguides 200, a p.sup.- -side electrode 201, an SiO.sub.2 layer 202, an InGaAsP gap 203, an InP cladding layer 204, an InGaAsP optical waveguide layer 205, an InP substrate 206, an n.sup.- -side electrode 207, diffusion regions 208, and a graded region (refractive index changing region) 209.
In the above optical modulator, the electrodes are formed in crossing regions of the two waveguides, and an electric field is applied to these electrodes to cause a change in refractive index, thereby shifting a light wave between the waveguides.
In the optical modulator shown in FIGS. 2A and 2B, light from an exit end of one of the waveguides is modulated. In the optical modulator of this type, however, although the degree of optical modulation is controlled by a change in refractive index, an absorption index is necessarily changed with the change in refractive index. Therefore, optical modulation cannot be stably performed, resulting in inconvenience. When the element length, i.e., the waveguide length, is shortened, and the optical modulator is designed to obtain a large change in refractive index with respect to a constant electric field so as to reduce a drive voltage, the range of wavelength to be modulated comes close to the range of wavelength corresponding to a large absorption index, resulting in inconvenience.
In a conventional wavelength division multiplexing system, a demultiplexer is used as a unit for dividing a given wavelength range into channels. Demultiplexing is performed by using a wavelength dispersion unit, e.g., an interference filter or grating to split light into transmitting and reflected components depending on wavelengths, or by utilizing different reflection angles. This demultiplexer has an advantage in that data of several wavelengths can be simultaneously received, but has a disadvantage in that the element area is increased because the data multiplexed in the wavelength region is split into a spatial region. In addition, in relation with the above drawback, the number of photodetectors to be integrated is limited. As a result, it is difficult to obtain a high-density wavelength multiplexing arrangement.
In order to solve the above problem, a variable wavelength filter is available to provide one photodetector which can sufficiently cope with a wavelength multiplexing scheme. In addition, when the number of channels of the variable wavelength filter is increased, the degree of wavelength multiplexing can be increased. This variable wavelength filter is exemplified as a filter utilizing a TE-TM mode converter, as proposed in Applied Physics Letters, Vol. 53, pp. 13-15 (1988). In Report OQE81-129, the Institute of Electronics, Information and Communication Engineers of Japan, (1981), a variable wavelength filter utilizing an even-odd mode converter is proposed. In addition, in Report US88-42, the Institute of Electronics, Information and Communication Engineers of Japan, (1988), a variable wavelength filter utilizing a surface acoustic wave (SAW) is proposed.
Although these variable wavelength filters have a wide variable wavelength range of 100 .ANG. or more, they are devices utilizing LiNbO.sub.3, thus posing a problem as to a coupling loss with a photodetector. In addition, since a refractive index is obtained by an electrooptical effect (i.e., a Pockels effect), a high voltage of several tens of V to 200 V is required. Furthermore, as variable wavelength filters using compound semiconductors such as GaAs and InP, filters utilizing a DFB (Distributed FeedBack) laser and a DBR (DistriButed Reflection) type laser at a value smaller than an oscillation threshold value are also known. In Report OQE88-65, the Institute of Electronics, Information and Communication Engineers of Japan (1988), a variable wavelength filter utilizing a Fabry-Perot laser at an oscillation threshold value or less is proposed.
These conventional variable wavelength filters can be easily integrated with photodetectors and have gains upon injection of currents. Since the variable wavelength range is directly determined by the width of a change in refractive index, a practical variable wavelength filter described above can obtain only a variable wavelength range of several .ANG. to several tens of .ANG..